Precise Measurements of Self-absorbed Rising Reverse Shock Emission from Gamma-ray Burst 221009A

TL;DR

This study presents unprecedented rapid, multi-frequency radio observations of GRB 221009A that capture an optically thick reverse-shock component rising in the radio band. By combining phenomenological light-curve fitting with an equipartition analysis, the authors constrain the RS-emitting region’s size, bulk Lorentz factor, and minimum internal energy within the first hours after the burst, and they track the RS self-absorption peak as it moves through the observing bands. X-ray data are used to contextualize the forward-shock component, while comparisons to RS theory reveal that simple thick-shell or thin-shell models alone cannot fully account for the observed decay and peak-evolution rates, implying forward-shock contamination and a complex circumburst environment. The results emphasize the critical role of rapid, dense, multi-frequency radio follow-up for probing the earliest jet physics in GRBs and outline concrete implications for future observing campaigns, including ambitious sub-mm predictions that could reveal RS signatures in other bursts.

Abstract

The deaths of massive stars are sometimes accompanied by the launch of highly relativistic and collimated jets. If the jet is pointed towards Earth, we observe a "prompt" gamma-ray burst due to internal shocks or magnetic reconnection events within the jet, followed by a long-lived broadband synchrotron afterglow as the jet interacts with the circum-burst material. While there is solid observational evidence that emission from multiple shocks contributes to the afterglow signature, detailed studies of the reverse shock, which travels back into the explosion ejecta, are hampered by a lack of early-time observations, particularly in the radio band. We present rapid follow-up radio observations of the exceptionally bright gamma-ray burst GRB 221009A which reveal an optically thick rising component from the reverse shock in unprecedented detail both temporally and in frequency space. From this, we are able to constrain the size, Lorentz factor, and internal energy of the outflow while providing accurate predictions for the location of the peak frequency of the reverse shock in the first few hours after the burst.

Figures (10)

• Figure 1: Radio light curves of GRB 22109A between $3$ and $18\,\textrm{GHz}$.(left) AMI--LA observations of GRB 221009A for the first 5 days post burst. Due to the high flux density in the first observation (between $T_{0}+3.1\,\rm{hr}$ and $T_{0}+7.1\,\rm{hr}$) we are able to split the data into $15\,\rm{min}$ time bins for each of the eight quick look spectral windows and derive flux density values directly from the complex visibilities. After the first day we derive fluxes from the image plane, using the top and bottom half of the AMI--LA observing band to monitor any spectral index evolution. See Methods for details on the data reduction process. (right) ATA observations of GRB 221009A for the first 5 days post burst showing an early-time peak most evident at $3$ and $5\,\rm{GHz}$ (and tentatively seen at $8\,\rm{GHz}$). All flux densities are derived from the image plane, see Methods for details on the data reduction process and imaging creation and processing. Error bars represent $1\sigma$ uncertainties.
• Figure 1: Very Large Array Sky Survey archival observations of the field of GRB 221009A. The Very Large Array Sky Survey (VLASS; version 2.2; lacy2020) observation of the field of GRB 221009A, with National Radio Astronomy Observatory Very Large Array Sky Survey (NVSS; condon1998) contours over-plotted in red. The restoring beam for the NVSS image is shown as a blue circle in the bottom left of the image, the restoring beam for VLASS is significantly smaller and is not shown, but has a major and minor axis of $3.31"$ and $2.29"$, respectively, at a position angle of $51.04^{\circ}$. The yellow circle is centred on the position of GRB 221009A atri_gcn and has a radius of $18"$. No significant emission from either survey is seen at the position of GRB 221009A. The most constraining limit is from VLASS for which we measure a root mean square three sigma upper limit of $\sim450\,\upmu\rm{Jy/beam}$. A number of deconvolution/calibration artefacts are present in the quick-look VLASS image and likely are the result of incomplete deconvolution of bright sources. These manifest as 'streaks' most notable between North and South on the East side of the image.
• Figure 1: An example image of GRB 221009A taken with the Arcminute Microkelvin Imager Large Array. The field of GRB 221009A observed with the AMI--LA. The pink cross marks the position of GRB 221009A (at the pointing centre) from which significant radio emission can be seen. Two field sources are evident to the west of the phase centre, corresponding to those discussed in the Archival Radio Observations section of the Methods and which are shown in Extended Data Figure \ref{['fig:archival_radio']}. The RMS noise in the image is $\approx60\,\mu\rm{Jy}/\rm{beam}$. The image range is between -0.5 mJy and 2 mJy, as indicated by the colour bar. The entire AMI--LA field of view is not shown. The restoring beam for this observation has a major and minor axis of $55"$ and $24"$, respectively, at a position angle of $5.6^{\circ}$.
• Figure 2: Early-time radio light curves and spectral index evolution of GRB 221009A.Top: Our first observation of GRB 221009A with the AMI--LA, separated into eight frequency channels and the flux density derived in $15\,\rm{min}$ time intervals as well the $3$ and $5\,\rm{GHz}$ light curves from the ATA. Bottom: The two-point spectral index $\alpha_{\nu_{1}}^{\nu_{2}}$ ($F_{\nu}\propto\nu^{\alpha_{\nu_{1}}^{\nu_{2}}}$, where $\nu_{1}$ and $\nu_{2}$ are the lower and upper frequencies used to calculate the two-point spectral index, respectively) measured between the highest and lowest of the eight AMI--LA quick-look frequency channels and the two ATA bands. Clear evolution can be seen throughout the observations, with the spectral index initially consistent with optically thick synchrotron ($\alpha\approx2.5$) and flattening with time. This is indicative of a break frequency (likely the self-absorption break) beginning to move into the AMI--LA observing band and then through the ATA observing bands. We mark the location of $\alpha=2.5$ and $\alpha=0$ with dashed horizontal lines to aid the reader. Error bars represent $1\sigma$ uncertainties.
• Figure 2: Power-law fits to our multi-frequency radio observations of GRB221009A. A broken power-law fit to each radio band where we observe a clear peak. The results of the fits are giving in Extended Data Table \ref{['tab:fitting']}. The rise and decay power law indices follow $F_{rise}\propto t^{1.34\pm0.02}$ and $F_{decay}\propto t^{-0.82\pm0.04}$, respectively. Error bars represent $1\sigma$ uncertainties.